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Thin-film optical filters are made by depositing alternating thin layers of materials with special optical properties onto a substrate, such as optical-grade glass. As light makes its way through the optical filter, its direction changes as it passes from one layer to the next, resulting in internal interference. This is due to the differences between the refractive indices of the materials in the dielectric thin-film coating. The configuration of the layers results in an optical filter that manipulates different wavelengths of light in different ways. Depending on the wavelength and type of optical filter, light can be reflected off of the filter, transmitted through it, or absorbed by it.

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Figure 1. Diagram illustrating the difference between single and multiple return signals from an aerial laser altimeter.
Image credit: Alluxa

LIDAR (Light Detection and Ranging) is a highly versatile active remote sensing technique that is used in Earth and atmospheric sciences, autonomous vehicles, urban planning, and many other applications. Some of the most important components of LIDAR sensors are the filters that isolate target signals, while preventing sunlight and other extraneous light from reaching the detector. A wide variety of applications and sensor types exist, from laser altimeters to Raman LIDAR systems, all with different return signal strengths and LIDAR filter requirements. Therefore, LIDAR filters must be designed with the specific application and sensor type in mind in order to maximize signal-to-noise ratio.

Fluorescence microscopes and imaging systems utilize fluorescent biomarkers and fluorescence filter sets to create bright, high contrast images of biomolecules, organelles, cells, tissues, organs, and organ systems. Because image quality is highly dependent on the design and overall performance of the fluorescence filters integrated into these systems, optical filter performance is just as important to the final image as sample preparation and fluorophore selection.

Used across a variety of biological disciplines, fluorescence-based flow cytometers rapidly and accurately quantify cells and cellular components. As one of the most important components of these systems, flow cytometry filters must be specifically designed to maximize signal-to-noise ratios while minimizing crosstalk between fluorophores.

The HELIX Spectral Analysis System has redefined measurement capabilities of high performance thin-film optical filters. HELIX is an instrument designed and developed by Alluxa Engineering staff to address the limitations of most commercially available spectrophotometers. The system’s capabilities are four-fold: it is able to track filter edges to OD7 (-70 dB), evaluate blocking to OD9 (-90 dB), resolve edges as steep as 0.4% relative to edge wavelength from 90% transmission to OD7, and resolve passbands that are as narrow as 0.1 nm at full width half maximum (FWHM).

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Arguably the most versatile active remote sensing technique, LIDAR (Light Detection and Ranging) is used across platforms and across disciplines. Long known to be one of the most important technologies in Earth and atmospheric sciences, LIDAR is now being utilized for obstacle avoidance in autonomous vehicles, urban planning, security, infrastructure development, and many other applications. This surge of novel uses recently forced an influx of technological advancements and a renewed interest in LIDAR sensors that is driving down the cost and making the technology more accessible.

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Some of the greatest recent advances seen in bio-imaging and detection are due to techniques that utilize non-linear optical (NLO) phenomena. These techniques have led to a Nobel prize, super-resolution images, label-free visualization of naturally occurring biomolecules, and greater freedom for working with in-vivo samples. Many NLO systems rely on the high peak pulse intensity of femtosecond lasers for signal generation. For this reason, the optical filters and mirrors integrated into these systems must have an appropriate laser damage rating, and the reflective components must be controlled for both group delay dispersion (GDD) and flatness. Choosing optical components that are specifically designed for NLO systems will ensure optimal signal strength, resolution, and image quality.

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Fluorescence based systems have revolutionized the way organisms, cells, and biomolecules are visualized and detected. However, challenges that are common in these instruments, such as bleedthrough, background autofluorescence, and poor signal-to-noise ratios (S/N), can reduce performance and lead to frustration.

Because system performance greatly depends on filter quality, optical filters are arguably the most important component of any fluorescence based instrument. With that in mind, here are some important concepts that should be considered when selecting optical filters for a fluorescence based system.

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Hard coated ultra-narrowband optical filters made using modern plasma processes offer much improved transmission, temperature stability and out of band blocking as compared to legacy soft coatings. These filters are used in optical systems as diverse as LIDAR (light detection and ranging), Doppler shift detection of plasma velocity, laser cleanup, chemical and gas sensing, as well as for cutting-edge astronomy and instrumentation applications.

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A novel computer-controlled deposition system for multicavity filters improves their spectral precision and contrast.

Narrowband filters are a critical technology for a variety of applications such as lidar (light detection and ranging), laser cleanup, chemical and gas sensing, instrumentation, and astronomy. The design principles are well known and relatively simple. All designs rely on stacked Fabry-Pérot resonant cavities with dielectric reflectors composed of layers a quarter of a wavelength thick, spaced apart by cavities multiple half-wavelengths across. Several cavity filters are used in combination to ‘square up’ the spectral wave shape, resulting in the transmitted light having a ‘flat-topped’ spectrum when compared with that from light passed through single-cavity filters, which has a sharp, peaked spectral shape. Such multicavity filters also have much steeper rejection responses than single-cavity filters: the less-steep spectral slopes of single-cavity filters can compromise signal-to-noise in narrowband detection.

Alluxa’s new class of Infrared (IR) bandpass and dual band filters bring the precision of dielectric filters to the IR between 2 and 5 microns. They are manufactured with an advanced, plasma enhanced PVD process using durable, hard metal-oxide, and durable semiconductor front surface thin films. The films have excellent environmental stability. The OH (water) band at 2.7 microns shows loss below the measurement limit even after 10 days of harsh Mil-810G temperature and humidity cycling.

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Alluxa Engineering Staff, December 2012

Due to their complex nature and diverse end-use applications, thin film optical filters have always been a challenge to specify. This paper defines the major types of filters and offers guidelines on how to specify key attributes, while also optimizing the cost.

Due to their complex nature and diverse end-use applications, thin film optical filters have always been a challenge to specify. This paper defines the major types of filters and offers guidelines on how to specify key attributes, while also optimizing the cost.

The filter types discussed include

Band pass types of ultra-square high cavity count, ultra-narrow and soft coating replacements

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Alluxa Introduces a New Line of Ultra-Flat Dichroic and Polychroic High Performance Thin Film Filters that Feature a Flatness of <0.1wave RMS Without the Need for Backside Compensation.

Alluxa is introducing a new line of thin substrate ultra-flat dichroics and polychroic filters for use in imaging applications that require flatness levels exceeding 0.1 waves RMS per inch. These filters are unique because they achieve flatness by eliminating the high stresses of as-deposited traditional ion-based coating process such as Ion Beam Sputtering (IBS) and Ion Assisted Deposition (IAD). Alluxa’s new technique uses a novel plasma coating process that produces low loss, fully dense dielectric films with essentially net zero stress on the primary coated side. This means that the coatings do not require industry standard backside compensation or the use of very thick and expensive substrates to assure flat surface figures. The backside is coated with a simple low cost and high performance Anti-Reflection (AR) coating. Alluxa has achieved flatness levels of <0.1 waves RMS at 632.8 nm per inch with substrates as thin as 0.5mm.

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Alluxa Engineering Staff, September 2012

Thin film optical filters with ultra-narrow bandwidth, high transmission and flat spectral profiles improve optical system performance for a variety of applications such as LIDAR, laser cleanup, and instrumentation.

Alluxa’s new class of ultra-narrow filters are designed for applications such as laser clean up, LIDAR, telecommunications and instrumentation and offer the narrowest and “squarest” filter profiles in the visible and NIR as well as transmission levels that approach 100%. They are manufactured with Alluxa’s proprietary advanced plasma enhanced PVD process using durable, hard metal-oxide, front surface thin films and are virtually impervious to environmental effects.

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Alluxa’s thin film optical filter technology has advanced to the point where the spectral slopes and blocking levels are challenging even the best metrology equipment and techniques. This paper discusses the issues and provides solutions to measuring the spectral response of this new class of high performance filters.

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Alluxa’s proprietary high speed plasma deposition technology for the first time delivers the optical performance and durability of hard coated optical filters at laminated soft coating pricing.

Soft coatings, also known as laminated coatings are well known to possess poor environmental durability, poor temperature stability, low transmission, high optical scatter and low blocking levels, yet they are still widely used due to their low cost, ready availability and legacy status. Alluxa’s proprietary high speed plasma deposition technology for the first time delivers the optical performance and durability of hard coated thin film optical filters at laminated soft coating pricing.